Procholeragenoid: a safe and effective antigen for oral immunization against experimental cholera

The immunogenicity and safety of procholeragenoid, a minimally toxic, heat-induced aggregate of cholera toxin (CT), were studied in enterically immunized rats and dogs. Although 99% less toxic than CT, procholeragenoid was only slightly less efficient in causing jejunal anti-CT responses in rats; in contrast, choleragenoid, the nontoxic B subunit pentamer of CT, was much less effective. The immunogenicity of procholeragenoid was due almost entirely to its large-molecular-weight components (MW = 10(6) to 10(7)) and was markedly reduced by preincubation with GM1 ganglioside or treatment with Formalin to eliminate residual toxicity. These findings suggest that molecular aggregation, binding to GM1 receptors on cell membranes, and stimulation of cellular adenylate cyclase each contributed to the effectiveness of procholeragenoid as a mucosal immunogen. In dogs, oral immunization with five 500-micrograms doses of procholeragenoid evoked vigorous anti-CT responses in jejunal mucosa without causing significant diarrhea. When subsequently challenged with virulent Vibrio cholerae, immunized dogs showed 83% protection against the development of severe or lethal diarrhea compared with non-immunized controls. These results confirm a protective role for mucosal antitoxin in experimental cholera and show that procholeragenoid is both safe and effective as an oral immunogen. Procholeragenoid, combined with other antigens of V. cholerae, may constitute a simple, safe, and effective oral vaccine for cholera.

Fine structural studies of ciliary processes after treatment with cholera toxin or its B subunit

Delivery of 2 micrograms of cholera toxin (CT), a specific, irreversible activator of adenyl cyclase, via the blood causes dilation of capillaries and stromal edema of the ciliary processes. These morphologic changes occur within 3 h, are maximal at 12 to 24 h, then gradually return to normal by 72 h. In the late phase of hypotony, ultrastructural changes in the ciliary epithelia, similar to Greeff vesicles, are due to a "paracentesis effect" from hypotony, caused by decreased aqueous flow through the eye. Delivery of 2 micrograms of the B subunit of CT (Sub-B) causes very mild capillary dilation and stromal edema of ciliary processes. These changes reach their peak at 3 h, then return to normal at 24 h. No significant damage occurred to the pigmented or non-pigmented epithelium with either agent. No hemorrhage, invasion of inflammatory cells or appearance of fibrin exudates in the ciliary processes could be detected.

The effect of stimulating somatic afferents on cholera secretion in the rat small intestine

The effect on rate of cholera secretion in the small bowel of activation of the group III and A delta afferent fibres in the sciatic nerve was studied in rats. Activation of these fibres at 3 Hz for 60 min significantly diminished choleraic secretion from 121 +/- 29 to 25 +/- 9 microliters x min-1 x 100 cm-2 serosal surface (mean +/- SE; n = 9). The effect was apparent after the nerve stimulation. Stimulation of the sciatic nerve had no significant effect on choleraic fluid secretion after interrupting the autonomic nerves to the intestine, nor did it significantly alter net fluid transport in non-choleraic intestine with intact nervous supply. It is proposed that the observations may explain the clinical reports of an effect of acupuncture on cholera secretion.

Oral immunization of dogs with purified cholera toxin, crude cholera toxin, or B subunit: evidence for synergistic protection by antitoxic and antibacterial mechanisms

The immunogenicity and safety of purified cholera toxin (CT), its B subunit, and a crude culture filtrate of toxigenic Vibrio cholerae (CrT) were compared in dogs immunized orally and challenged with virulent V. cholerae. CT and CrT caused marked protection in two- or three-dose regimens. Protection due to CT occurred only with doses that caused transient, sometimes severe, diarrhea in most dogs; this protection was proportional to the peak antitoxin response in jejunal mucosa and lasted at least 15 weeks. In contrast, minimum protective doses of CrT contained much less cholera toxin, caused very mild diarrhea in only 21% of the dogs, and evoked protection that was greater than predicted from the modest jejunal antitoxin response. B subunit caused smaller jejunal antitoxin responses than did similar doses of CT and was poorly protective, the 50% protective dose being >40-fold greater than that of CT. Two observations indicated that protection due to CrT involved synergy between antibacterial and antitoxic immune responses. First, the 50% protective dose of CrT was 24-fold and >36-fold smaller than the 50% protective doses of its CT and non-CT antigenic components, respectively, when tested separately. Second, protection was greater in CrT-immunized dogs than in CT-immunized dogs for a given mucosal antitoxin response. Low doses of CrT evoked serotype-specific protection, indicating that the serotype-specific O somatic antigen contributed significatly to antibacterial protection. These results suggest that a simple, effective, nonliving oral vaccine for cholera based on combined antibacterial and antitoxic immunity can probably be achieved. However, further studies are needed to determine how a protective antitoxic response can be evoked without causing diarrhea during immunization.

Intestinal antibody responses after immunisation with cholera B subunit

In about 80% of Bangladeshi volunteers a single oral or intramuscular immunisation with a new cholera toxoid immunogen (B subunit) gave rise to a local intestinal secretory immunoglobulin A (IgA) antitoxin response as measured in intestinal-lavage fluid by enzyme-linked immunosorbent assay methods. The rise in IgA antitoxin titre was similar for both immunisation routes and was comparable to that seen after clinical cholera; however, the response persisted longer after oral than intramuscular immunisation. A second immunisation by either route evoked an antitoxin response which usually closely resembled that seen after the first immunisation.

Intraocular pressure and aqueous flow are decreased by cholera toxin

Delivery of 2.1 microgram of cholera toxin, a specific, irreversible activator of adenylate cyclase, via the blood lowers IOP from 17.4 to 11.2 mm Hg in 81/2 hr. decreases net aqueous flow by about 50% in 8 hr, and doubles blood flow to the anterior uvea at 8 to 13 hr. Intravitreal injection of 0.26 microgram of cholera toxin lowered IOP from 15.0 to 9.6 mm Hg, but heat-inactivated toxin had no effect on IOP. The toxin activates adenylate cyclase from ciliary processes 2.2-fold and stimulates cyclic AMP production by ciliary processes 7.4 times. Absence of aqueous flare, normal protein concentrations in the aqueous, and histologic examination all confirmed the functional and structural integrity of the blood-aqueous barrier after cholera toxin infusion. The data point to an important role for ciliary process adenylate cyclase in regulation of aqueous flow and maintenance of IOP.

Changes in intestinal fluid transport and immune responses to enterotoxins due to concomitant parasitic infection

The effect of a parasitic infection on enterotoxic diarrhea and on local and systemic formation of antibody to the toxin after immunization was studied in mice. Trichinella spiralis infection was chosen as the model, since the effects of the parasite when residing in both intestinal and extraintestinal sites can be studied. It was found that during the intestinal stage of the infection, the fluid response to cholera toxin as well as dibutyryl-cyclic adenosine 3',5' -monophosphate was greatly enhanced and that this was associated with a marked reduction in the absorption of fluid from the intestine. Later in the infection (migration stage), fluid accumulation in response to cholera toxin was significantly reduced, whereas absorption was normal and secretion in response to dibutyryl-cyclic adenosine 3',5'-monophosphate was somewhat increased. Still later in the infection (muscular stage), the fluid-secretory response to cholera toxin was normal. There was a drastic depression of local formation of antitoxin of both immunoglobulin and immunoglobulin classes in mice given the first two of four oral immunizations with cholera toxin during the intestinal stage of T. spiralis infection. When the priming was given before or after the intestinal stage, the local antitoxin response was not affected. The titers of circulating antibodies were also depressed in mice given the first immunizations during the intestinal stage. In addition, significant though less pronounced depression of the serum antibody response was observed in mice primed during the extraintestinal stage.

Intestinal immune response to cholera toxin: dependence on route and dosage of antigen for priming and boosting

The influence in immunization with cholera toxin of the route and antigen dose on intestinal antibody formation and protective immunity against experimental cholera was studied in mice. Administration by either the intravenous or oral route induced effective priming as well as boosting of mucosal immunity, with the effects on intestinal immunoglobulin A antitoxin synthesis and protective antitoxic immunity showing excellent concordance. A strong antigen dose dependence was found for both priming and boosting of the local immunity, irrespective of route. Very efficient high-dose priming did, however, partially decrease the dose dependence of the booster response and, conversely, a high booster dose partly overcame the relative inefficiency of low-dose priming. The results suggest that the amount of antigen reaching the immunocompetent cells in the gut rather than the route of administration per se determines the mucosal immunizing effect.

Antitoxic immunity to cholera in dogs immunized orally with cholera toxin

Colera toxin was evaluated as an oral immunogen against experimental canine cholera. Dogs were immunized orally with 100-microgram doses of purified cholera toxin or comparable doses of crude toxin. Both doses caused moderate diarrhea in most nonimmune dogs. Repeated oral doses (12 doses in 54 days) gave marked protection against the diarrheal effect of oral toxin, provoked a vigorous antitoxic response in jejunal mucosa, and gave nearly complete protection against subsequent oral challenge with living virulent Vibrio cholerae. Protection appeared to be due largely to the antitoxic response in intestinal mucosa. The effectiveness of cholera toxin as an oral vaccine contrasts with the previously described ineffectiveness of toxoid given orally. This study provides an example of mucosal immunity due to a nonreplicating vaccine given orally and suggests that cholera toxin may be useful as a component of an oral vaccine for cholera.

Protection against experimental cholera by oral or parenteral immunization

Comparisons were made between the antigenic potency and protective capacity of several cholera toxin derivatives. Rabbits were immunized parenterally with 50 microgram of cholera toxin, A subunit, B subunit, procholeragenoid, or Wyeth glutaraldehyde toxoid 20101. Examination of the antibody response curves revealed that cholera toxin elicited serum antitoxin responses that rose more quickly than in the subunit-immunized animals; however, antitoxin levels were of the same magnitude after 10 weeks. Parenteral immunization with procholeragenoid evoked antibody titers that were similar to the toxin, whereas Wyeth toxoid yielded only one-tenth the level of antitoxin. Oral immunization with procholeragenoid as well as Wyeth toxoid resulted in lower serum antitoxin titers than that achieved with parenteral immunization, despite the oral administration of 10 times the parenteral dose. Analysis of protection against live-cell challenge revealed that parenteral administration of procholeragenoid provided the best protection against fluid accumulation. Oral immunization with procholeragenoid also was very effective, whereas oral immunization with B subunit or Wyeth toxoid resulted in minimal protection. Also, the A subunit provided surprisingly more protection than did cholera toxin.

Effect of intra-arterial injection of crude cholera enterotoxin on canine small bowel

Injection of a crude preparation of cholera enterotoxin into the superior mesenteric artery caused isotonic fluid secretion by the canine small bowel. In dogs previously exposed to cellular antigens of Vibrio cholerae, the time course of the intestinal response to intra-arterial injection of crude enterotoxin was similar to that observed after intraluminal exterotoxin challenge.

Suppression of local intestinal immunoglobulin A immune response to cholera toxin by subcutaneous administration of cholera toxoids

Cholera toxin has been shown to modulate immune responses, generally producing enhancement when administered simultaneously with antigen and suppression when administered a day or more earlier. In a previous study using chronically isolated ileal loops in rabbits, we found that two subcutaneous (s.c.) "priming" and "boosting" doses of biologically active cholera toxin suppressed the local intestinal immunoglobulin A response to intraloop doses of cholera toxin. In the study reported here, two different biologically inactive but antigenically intact cholera toxoids, glutaraldehyde toxoid and choleragenoid, where administered s.c. by the same immunization schedule as for toxin in the earlier experiment. Suppression of local immune response to intraloop cholera toxin as compared with animals receiving no s.c. inoculations was again found. The results suggest that in this model suppression was immunological (mediated by an immunological mechanism) rather than toxigenic (mediated by biological activity of cholera toxin). In addition, the occurrence of suppression of local intestinal immune response after systemic immunization suggests that suboptimal protection against enteric infections could occur after s.c. vaccination.

Immunity of cholera in man: relative role of antibacterial versus antitoxic immunity

Purified cholera toxoid is antigenic when given enterally and orally. Purified toxoid fails to provide protection against experimental challenge. Clinical cholera confers formidable protection against homologous or heterologous rechallenge. Failure to culture vibrios from intestinal fluid or stool of re-challenge volunteers suggests that the predominant immune mechanism is antibacterial rather than antitoxic.

Increased adenylate cyclase activity and rapid weight loss following intraseptal injection of cholera toxin

Cholera toxin, stereotaxically injected into the medial septal nucleus of the rat, leads within 24 h to a dramatic decrease in body weight and an increase in septal adenylate cyclase activity. Toxin-treated rats drink one-third the water of vehicle-treated animals while excreting two-and-one half times the urine. Food intake over the 24-h period is depressed to 13% of control but feces production was normal. The dramatic increase of urinary output suggests that cholera toxin activates a septal adenylate cyclase system which supressess the release of antidiuretic hormone. Cholera toxin injection into the septum may be a unique alternative to electrical stimulation for investigating septal involvement in the regulation of neuronal and metabolic processes.

[Role of mediators in the quantitative and qualitative changes in the intestinal contents of rabbits with experimental cholera intoxication]

Experimental cholera intoxication caused in 40 rabbits (control--40 rabbits) by intravenous injection of cholerogen was accompanied by disturbance of the mediator balance in the blood plasma and the intestinal wall, chiefly in the samll intestine. Against the background of an increased catecholamine content in the intestinal wall the amount of fluid containing mostly the sodium and potassium bicarbonates and chlorides increased in the lumer of its corresponding sections. The detected regularity possibly served as one of the pathogenetic factors in the mechanism of fluid accumulation in the intestine under the effect of cholerogen.

Protective antitoxic cholera immunity in mice: influence of route and number of immunizations and mode of action of protective antibodies

An adult mouse model has been elaborated for studies of experimental cholera (Vibrio cholerae enterotoxin-induced intestinal secretion) and protective antitoxic immunity in either ligated small bowel loops or intact small intestine. Mice of different inbred strains varied markedly in intestinal susceptibility to toxin, C57B1 being the most sensitive strain tested. Fluid accumulation started within 1 h after the inoculation of toxin and was maximal after 5 h, whereafter recovery gradually ensued. The dose-response curve was sigmoid, the ED50 of crude toxin being equivalent to about 0.1 microgram purified toxin/cm in the loops and 0.3 microgram/cm in the nonligated intestine. Two peroral (p.o.) immunizations induced significant protective immunity which increased markedly after two further immunizations by the same route. Additional p.o. immunizations did not appreciably enhance the protective immunity any further. Intravenous (i.v.) vaccination had to be repeated more than 5 times before intestinal immunity could be observed. No correlation between serum antitoxin titers and protective immunity was found. Electron microscopic examination revealed that whereas peroxidase-coupled cholera toxin bound tightly to intestinal microvilli from unimmunized or 5-times i.v. immunized mice it did not bind to the microvilli of p.o. immunized animals. The data thus suggest that the protective immunity is mediated exclusively by locally produced antibodies which prevent the binding to toxin to the gut epithelium.

Correlation between intestinal synthesis of specific immunoglobulin A and protection against experimental cholera in mice

The importance of locally and systemically formed antibodies of various classes for protection against experimental cholera has been studied in mice immunized with cholera toxin. Groups of mice were given various numbers of peroral or intravenous immunizations, or a combination of both. Serum antibodies and antibodies synthesized by spleen and small intestine in vitro during tissue culture were measured by the enzyme-linked immunosorbent assay, and protective immunity against intestinal toxin challenge was determined by means of a small-bowel loop assay. Regression analyses showed a close correlation between the magnitude of intestinal synthesis of specific immunoglobulin A (IgA) antibodies and protection (r = 0.98), whereas neither the local formation of IgG or IgM nor the production of antitoxin antibodies of any immunoglobulin class by spleen showed any significant correlation with protection. The serum titers of IgG and IgM antibodies did not show any such relation, whereas the level of specific IgA in serum, probably mainly derived from the intestine, correlated significantly (r = 0.90).